1. Field of the Invention
This invention relates to polyurethane compositions with high tear strength and the process for preparation thereof. In particular this invention is concerned with improvement of the tear strength of polyurethane composition by incorporation of finely divided acidic silica particles in a predetermined ratio with isocyanate-terminated prepolymer.
2. Description of the Prior Art
The tear strength (hereinafter referred to as .GAMMA.) of the prior polyurethane elastomer is mostly in the range of 10 - 50 kg/cm and this level is satisfactory only for the limited applications for which the polyurethane composition has been used.
However, the conventional polyurethane elastomer composition can not be used for severe applications such as for tire tread used on rough roads, cover material for golf balls durable to high impact tear stress, and rubber screen used for sieving crushed stone or ore. Whereas it was difficult to improve further the tear strength of the prior polyurethane elastomer composition which had already advanced to have a comparatively high level of tear strength, the improvement if attained will develop further applicable fields of the polyurethane elastomer composition, including the above mentioned applications. While the incorporation of silica particles so as to improve tear strength is well known in the field of rubber, and plastic, other than polyurethane, it has been understood that the polyurethane composition which has a micro heterogeneous reinforcing structure will result in insufficient effect of the silica incorporation and will results in some drop of mechanical strength.